1. Field of the Invention
The invention relates to controls and monitors for oil well rod pumps and similar cyclic loads. In particular, the rate at which the underground pump chamber refills in the interval between power strokes of a rod pump is determined indirectly by monitoring variations in electrical loading of the pump motor at particular phase regions in the cycle of the pump. The phase position of the pump cycle is referenced by power peaks or zero crossings of the cyclic load. A controller having a digital processor samples the electrical loading during the pump cycle. Variations in the extent of pump chamber refilling preferably are determined by examining the cycle by cycle differences in the power level of the pump during the downstroke, when the pump rod travels freely through air until encountering fluid.
2. Prior Art
Oil well walking beam pumps extract fluid from a downhole pump chamber by repeatedly raising and lowering a series of steel rods coupling the downhole pump and the surface beam pumper assembly. The repetitive raising and lowering of the steel rods causes a piston in the downhole pump assembly to pull the well fluids to the surface.
The surface beam pumper assembly typically includes a rocking beam with one end coupled to a pump motor by a crank assembly. The crank assembly has a counterweight intended to balance the loading of the motor by offsetting at least part of the weight of the pump connecting rods which are cantilevered on the opposite end of the rocking beam. Nevertheless, as the rods to the downhole pump are raised and lowered, the loading of the motor passes through a cycle during which potential energy is stored as the pump rods are lifted, and released as the pump rods are lowered.
The motor is typically an electric motor that is geared down to accommodate the relatively low frequency of the pump stroke. A three phase motor is typical. Motor and circuit protection contactor devices typically are provided for breaking the motor circuit in the event of a short circuit or motor overload. Additionally, a controller that is responsive to conditions in the well may be coupled to the contactor devices, for example to operate the pump intermittently at a rate that can be supported by the geological formation. The controller or the contactor device itself may include means for measuring the current in the motor circuit and/or the line voltage by analog or digital circuits, as a part of the circuit protection function, as well as to vary the operation of the pump to suit conditions at the best efficiency.
It is known to provide a contactor for an oil well with relay contacts that rearrange the line couplings of a three phase motor when current loading conditions indicate that the pump is operating inefficiently, for example as disclosed in U.S. Pat. No. 4,220,440--Taylor et al. U.S. Pat. No. 4,695,779--Yates discloses a similar controller that includes a processor and a number of timers that switch between operational modes upon the occurrence of distinct stall conditions.
A processor with a range of flow and energy consumption sensors for assessing well operation is disclosed in U.S. Pat. No. 4,767,280--Markuson, and a processor that integrates additional factors such as the proportions of oil and water in the recovered fluid is disclosed in U.S. Pat. No. 5,070,725--Cox et al.
Although the invention is described herein primarily with reference to a walking beam pump, it is also possible to apply the concepts of a walking beam pump to other forms of cyclic loads. U.S. Pat. Nos. 4,601,640 and 4,493,613, both to Sommer, for example, disclose a compact pump arrangement that reciprocates a piston but does not employ a beam. Instead, a reversing motor manipulates the piston via a cable. These, and the foregoing U.S. Patent disclosures are hereby incorporated by reference, for their teachings of well motor control and sensing arrangements.
Wells are frequently instrumented for purposes of assessing operational parameters. The fluid flow rate produced by the well is an advantageous parameter to measure, and can be measured using flow rate sensors at any point along the conduits through which the fluid is pumped. The fluid pressures produced in the well by the pump can also be monitored, and used to develop additional information, such as the rate at which the geological formation is refilling the pump, and other aspects of well performance. One means for sensing well fluid pressure indirectly is to sense tension and compression of the moving pump structures, for example using strain gauges mounted on such structures or load cells coupled between them.
There are a number of aspects of well and/or pump performance that are pertinent to issues of efficiency, maintenance, capacity, switching between operational modes and the like. The object for the well is of course to supply the maximum fluid possible, and preferably to maximize the percentage of the fluid that is oil rather than water or mud while minimizing the power consumption of the pump. However, optimizing pump operation requires that the operation of the pump be varied to suit conditions. A monitoring system and controller can be provided to sense conditions and to adjust operational parameters such as the frequency of cyclic operation, the manner in which power is coupled to the motor windings and so forth.
The amount of useful work that a fluid transport device performs is the product of the mass rate of fluid flow and the pressure differential or elevation head. The total head borne by the pump includes static and dynamic factors such as the discharge head and the suction head maintained, a velocity head, frictional resistance, etc. The variations in a number of these factors, especially fluid pressure and fluid flow, is cyclic due to the cyclic operation of the pump. It is therefore necessary to assess fluid pressure and flow information as a function of the point at which such data is sampled in the periodic cycle of the pump. The monitoring and control system of the pump thus requires the input of information on the present phase angle of the pump.
The phase angle of the pump can be measured by more or less sophisticated means. For example, a limit switch can be mounted for repetitive operation by contact with the pump beam, and used to trigger sampling of process data at the same point during every cycle, or between counted cycles. A shaft angle encoder can be mounted to produce pulses with angular displacement of the beam or of the motor crank, etc., which allows measurements to be taken at defined points in the cycle. These devices require proper setup and maintenance, and can suffer from mechanical failure. Thus the known arrangements are expensive both initially and with continuing maintenance and use.
It is possible in the operation of a rod pump or the like to run the pump at a frequency that exceeds the rate at which the geological formation can refill the pump chamber. Should the pump be operated at too high a rate, part of the electrical energy expending in reciprocating the pump is wasted because each stroke lifts only a portion of the full stroke capacity of the pump. Inefficient operation of the pump in this manner is characterized by a greater expenditure of energy in friction, per unit of fluid pumped, than would occur if the pump chamber was refilled between strokes and each stroke lifted a full stroke capacity volume of fluid.
When the pump rod begins a down stroke, the pump motor must raise the counterweights using electrical power if the well is full. If the well is pumped off (not yet refilled), the pump rod travels through air on the down stroke until the fluid level is reached. Gravity and the combined weight of the pump rod and the fluid carried in the pump rod assist in raising the counterweights during this free fall of the pump rod. When the surface of the fluid is reached by the pump rod, the motor power is greater. It is possible to mount strain gages or load cells as well as rod position sensors, to determine when the pump is not refilling at the same rate the fluid is being pumped out. However, such measurements are complex and the sensors and connections are subject to mechanical stress.
It would be advantageous to provide a device that can determine information needed for assessing or controlling this aspect of pump operation without such components. The present invention is arranged to develop information on the filling rate of a rod pump indirectly from variation in the loading of the pump motor. It has been discovered that the electric power consumption of the pump motor in the free-fall condition characteristic of incomplete pump filling is reduced to about one half of the power consumption in the normal condition when the pump is full. By collecting and examining sampled data representing electrical power consumption, as referenced to a phase angle determined during each pump cycle from the point of minimum instantaneous power consumption, the invention detects the fillage of the pump. The fillage data can be reported or used to signal a pump-off controller to reduce the rate of operation of the pump or to assume an inactive state to permit the geological formation time to recover.